This invention relates to perimeters for visual field testing, and more particularly pertains to an improved image projection system and an improved fixation monitoring system for such devices.
Many modern perimeters for testing the visual field, whether manual or automatic, consist of a hemispheric bowl or observation hemisphere which is evenly illuminated. The patient looks at a central fixation point while spots of light or targets of various sizes and intensities are projected at various places within the observation hemisphere and the patient indicates whether or not the target is visible. The field of view is then mapped out point-by-point on a chart, sometimes with the aid of a computer.
Visual field testing involves not only the determination of the extreme outside boundary of the visual field but the identification of any regions of relative blindness within that field. Testing usually includes static threshold perimetry and kinetic or dynamic perimetry. In static threshold perimetry, a location is chosen and the target is presented at that location without moving it. The target is increased in intensity (or size) until it becomes visible, determining the visual threshold at that location. This process is then repeated at other points. In kinetic or dynamic perimetry, a selected target is moved from an area where it is not seen toward an area where it is seen and the location where the target becomes visible is recorded. This process is repeated from all directions toward the center (fixation) until an isopter or line can be drawn connecting the points where the target first became visible. Several isopters can be drawn by repeating the process with several targets of differing intensity or size. Since it is normal for a test stimulus to be seen everywhere within the region bounded by an isopter, the region can be scanned to locate areas where the target is not seen, i.e., the physiologic blind spot and areas of relative blindness. To conduct such testing utilizing spot stimuli is laborious, suffers from inaccuracies and is time consuming.
Obviously the validity of these tests depends on the patient correctly reporting what he sees or does not see. During the examination there is an urge for the patient to look at the target in order to see it better. Since a data point must be rejected if fixation was not established at the instant the data point was taken, the examiner has to constantly check that the patient's eye remains fixed on the central fixation point. Obviously, it is no good for the examiner to check fixation after the patient responds. The patient may have looked at the target and responded, and regained fixation before the examiner checks. Typically fixation is checked by the examiner observing the pupil of the patient's eye through a telescope which is located behind the fixation point. Recently electronic motion detectors have been used to check eye fixation automatically.
Vision field testing is a painstaking procedure for the patient as well as the examiner and the cooperation of the patient diminishes after a while and is sometimes lost altogether.
Thus, it is desirable to provide a perimeter which projects an effective target across the assumed viewing range of the patient and which prevents recording of invalid data.